I dont think you can decrease inflation by putting a set number to the heirloom sell.
Even if you could set a standard price, remember that ppl still keep gaining money, which would just inflate the prices in the market.
Also, heirloom selling is a nice way for a new player to make some money and close the gap with a long time player. I know this well, being a 2nd gen, without selling my loom it would have took me weeks before being able to afford anything +2 (since i did about 60k my 1st gen, from upkeep and so).
Selling a loom for 600k gives you almost a +2 item or almost 2 items +1 in the market, if you are buying low.